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Supplementary Material
Structural analysis of CdCl2 solvation with one to six water molecules:
When a single water molecule is added around CdCl2 the optimized geometry looks like the
one computed by Castro et al. [18] for HgCl2. An interaction is also found between Cd and
the water oxygen but in this case the Cd-Ow distance is 2.398 Å, significantly shorter than the
2.68 Å Hg-Ow distance found for HgCl 2; this fact is due to the much smaller core of Cd as
compared with Hg. Also note that the Cl-Cd-Cl (165°) and Cl-Cd-Ow (79°) angles are both
smaller than in the HgCl 2 case and the water molecule is symmetrically located with respect
to both Cl ligands. As for the HgCl2-H2O complex, the nature of the Cd-Ow interaction is
electrostatic [18], with an oxygen lone pair pointing towards cadmium while a water hydrogen
is slightly tilted towards one of the chlorine atoms. With two water molecules the optimized
structure resembles again to the one computed [18] for HgCl2-(H2O)2. However, a much
larger deviation from linearity of the solute is found since the Cl-Cd-Cl angle is only 155° and
the Cd-Ow distance decreases to 2.30 Å.
The three-water structure also resembles the one obtained for HgCl2–(H2O)3 [18]. At this
stage only one direct Cd-Ow interaction appears, at 2.24 Å, leading to a tri-coordinated
cadmium atom. The Cl-Cd-Cl angle is now 152° and the symmetric Cd-Cl distances
increase to 2.35 Å. In the optimized structure with four water molecules three of them are
stabilized by two hydrogen and halogen bonds, while the fourth is directly coordinated to
Cd. The optimized Cs structure has a symmetry plane that passes through two “equatorial”
oxygens directly coordinated to Cd and themetal atom. Two Cd-Ow interactions are found
at 2.24 and 2.42 Å, the former corresponding to the central hydrogen-bonded water. With
four water molecules the Cd-Cl bonds are significantly stretched to 2.39 Å from its isolated
molecule value (2.27 Å) and the Cl-Cd-Cl angle further decreases to 149°. Addition of the
fifth water molecule leads to a pentacoordinated Cd atom in a square pyramid (two water
oxygens and two Cl atoms at the base) configuration around Cd, with three water oxygens
directly interacting with the metal atom. The optimized C s structure has a symmetry plane
that is conserved from the previously optimized tetrahydrated complex; the Cl-Cd-Cl angle
drastically decreases to 143° from the free solute value and the Cd-Cl distances become
even longer, at 2.45 Å. The Cd-Ow distances are 2.23, 2.42 and 2.46 Å, the former
corresponding to the twice-hydrogen bonded water molecule located on top of the nearsquare pyramid. Pentacoordinated Cd is found only for these two cases. The Cd-Ow
distance decreases as the number of hydrogen bonds increases for the Cd-coordinated
water molecules. Note that the optimized structures solvated with three, four and five
molecules all belong to the Cs point group. Addition of a sixth water molecule leaves
unaltered the coordination around CdCl 2 but the incoming molecule establishes one
hydrogen bond with a water molecule and a halogen bond. The Cd-Ow distance decreases
as the number of hydrogen bonds increases for the Cd-coordinated water molecules (2.48,
2.31 and 2.23 Å for zero, one and two hydrogen bonds, respectively).
MP2/AVDZ harmonic frequencies of selected vibrational (S/A for symmetric/antisymmetric
stretching, B for bending) modes of gaseous and microsolvated CdCl2-(H2O)n systems. The
water oxygen atoms (Ow) are those directly linked to Cd. Cl-Cd-Ow is the angle concerning
the water oxygen closest to Hg. All energies in wavenumbers.
n
0
S
A
B
Cd-Ow
Cd-Ow
336
420
81
1
313
394
87
236
2
306
379
83
323
3
298
363
74
349
4
288
341
69
339
246
5
282
308
67
346
242
a
21
232
259
50
378, 387 236
a
Computed at the B3PW91/6-31G** level
Cl-Cd-Ow
256
249
Energetics
B3PW91 absolute (a.u.) and water binding free energies (kcal/mol) of the CdCl2–(H2O)n
structures at 298K. Basis sets are aug-RECP(Cd), cc-pVTZ(Cl) and 6-31G(d,p) for O and H
atoms.
a
n
G2980
ΔG2980
0
-197.993168
1
-1164.704823
2
-1241.100086 -11.40
3
-1317.493391 -14.34
4
-1393.889544 -19.06
5
-1470.284055 -22.75
6
-1546.677579 -25.82
7
-1623.069835b -28.10
-1623.066374c -26.01
8
-1699.459000 -28.44
12
-2005.027978
-37.51
17
-2386.983535
-45.31
21
-2692.550472
-53.10
24
c
-7.24
-2921.725818 -77.78
-2921.718653b -72.10
a
ΔG2980= G2980(system optimized with n water molecules) - G2980(CdCl2)- n G2980(H2O), where
G2980(H2O)= -76.388628 a.u
b
Free energy of the pentacoordinated structure
c
Free energy of the hexacoordinated structure
Vectors showing the imaginary mode of the PCM-optimized structure for CdCl 2-(H2O)5